Laser beam scanning apparatus

ABSTRACT

A laser beam scanning apparatus for beam scanning a medium with the laser beam being deflected by a deflector, comprising a first mirror provided for reflecting the laser beam after deflected by the deflector and a second mirror provided for folding a light path between the first mirror and the medium in such a relationship that a first light path between the deflector and the first mirror is not intersected with a second light path between the second mirror and the medium. The first and second mirrors are arranged so as to be movable in parallel respectively for adjusting a length of scanning line on the recording medium, wherein either one of the first and second mirrors is moved when adjusting and said moved mirror has a lesser shift amount of the scanning line on the recording medium in a direction perpendicular to the scanning line as compared with the other mirror.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a laser beam scanning apparatus and,more specifically, to a laser beam scanning aPParatus in which a laserbeam from a laser beam generating source is deflected by a deflector,whereby a recording medium is scanned and an image is formed thereon.

2. Description of the Prior Art

Generally, optical apparatuses for printers employing a laser beam forcarrying image information such as shown in Japanese Laid-Open PatentApplication Nos. SHO 61-3114 and SHO 58-93026 are well known. In opticalapparatuses of the above-mentioned type, a distance between a deflectorand a recording medium is determined by the width of an image, saiddistance being over 200 mm for A4 size in general. An image reflectingmirror is used for making such a long optical path compact. The elementsconstituting the optical apparatus should preferably be provided as aunit, so as to facilitate the positioning of the apparatus to the mainbody of the printer, to reduce vibration and to facilitate the care ofthe apparatus such as exchange of parts.

On the other hand, compared with an electrophotographic copying machine(plane paper copier) utilizing projected images of originals by visiblelight, higher image quality is required for a laser beam printer.Therefore, the optical apparatus is a critical portion dominating theimage quality, and various portions thereof should be adjusted whenassembled.

Especially, the adjustments of the length of scanning line, which is alength of a laser beam track on the recording medium as well as of anilluminating position on the recording medium are important. Examples ofmethods for adjusting the scanning line length include one wherein theentire optical unit is moved relative to the recording medium, and onewherein the image reflecting mirror is moved in the direction of a lightaxis and the like. Compared with the former method, the latter onefacilitates the adjustment as a optical unit and is preferable in viewof the structure as a unit.

However, the movement of the image reflecting mirror for adjusting thescanning line length causes a problem in that the illuminating positionon the recording medium is interlockingly altered. The change of theilluminating position caused by adjusting the scanning line lengthbecomes a problem in the case of adjusting the scanning line lengthafter the adjustment of the illuminating position or adjusting thescanning line length with the illuminating position fixed.

SUMMARY OF THE INVENTION

A main object of the present invention is to provide a laser beamscanning apparatus wherein an amount of change of the illuminatingposition can be made within a permissible range relative to an amount ofadjustment for a scanning line length.

This and other objects of the invention can be accomplished by providinga laser beam scanning apparatus for beam scanning a medium with thelaser beam being deflected by a deflector, said apparatus comprising:

a first mirror provided for reflecting the laser beam after beingdeflected by the deflector;

a second mirror provided for folding a light path between the firstmirror and the medium in such a relationship that a first light pathbetween the deflector and the first mirror is not intersected with asecond light path between the second mirror and the medium; and

the first and second mirror; being arranged so as to fulfill thefollowing conditions:

    0<θ.sub.1 <45°

    -θ.sub.1 <θ.sub.2 <90°-2θ.sub.1 <90°,

where θ₁ and θ₂ represent angles between the mirror surfaces thereof anda plane perpendicular to a beam scanning plane from the deflector to thefirst mirror, and also to be movable in parallel, respectively, foradjusting a length of scanning line on the medium, wherein either one ofthe first and second mirrors is moved when adjusting and said movedmirror has lesser amount of the scanning line on the recording a mediumin a direction perpendicular to the scanning line as compared with theother mirror.

These and other objects, advantages and features of the invention willbecome apparent from the following description thereof taken inconjunction with the accompanying drawings which illustrate specificembodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following description, like parts are designated by likereference numbers throughout the several drawings.

FIG. 1 is a perspective view showing the basic structure of an opticalapparatus in accordance with the present invention;

FIG. 2 is a plan view of the main portion of the optical apparatus;

FIG. 3 is a vertical sectional view of FIG. 2;

FIG. 4 is a schematic diagram illustrating the relation between thescanning line length, the point of focus and the angle of the beam;

FIG. 5 is explanatory view illustrating angular relation of the beamreflected by first and second mirrors;

FIGS. 6(A) and (B) are explanatory views illustrating the change of alight path when a first mirror is moved;

FIGS. 7(A) and (B) are an explanatory views illustrating the change of alight path when a second mirror is moved; and

FIGS. 8 to 12 are graphs showing relations between angles θ₁ and θ₂.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the present invention will be hereinafter describedwith reference to the figures.

In the present optical apparatus, a laser beam carrying imageinformation irradiates a photoreceptor provided on a surface of aphotoreceptor drum 1, whereby the drum is scanned by the laser beam inthe axial direction of the drum 1. As shown in FIG. 1, the apparatuscomprises a semiconductor laser 10, a collimator lens 11, a polygonmirror 12, a fθ lens 13, image reflecting mirrors 14 and 15 and a sensorfor detecting a start position for image formation (hereinafter referredto as SOS sensor) 20, and mirror 21 used only for the sensor shown inFIGS. 2 and 3, and the apparatus is provided on a substrate 30 as aunit.

A laser beam emitted from the semiconductor laser 10 is made into aparallel light by the collimator lens 11 to be guided to a polygonmirror 12. The polygon mirror 12 is rotatively driven by a motor 16. Bymeans of this rotation, the laser beam is deflected in the planeorthogonal to the axis of rotation. The defected laser beam is projectedonto the photoreceptor drum 1 through the fθ lens 13 and the mirrors 14and 15 to thereby be scanned on the drum 1. The fθ lens 13 equalizes thescanning speed of the laser beam on the drum 1 through beam scanning inassociation with the rotation of the polygon mirror 12.

The SOS sensor 20 compensates the error of the recording position foreach scanning line derived from the division error of the deflectionplanes of the polygon mirror 12. In the SOS sensor 20, the laser beamreflected by the first image reflecting mirror 14 is reflected by themirror 21 used only for the sensor, and thereafter, it is againreflected by the mirror 14 to enter the light receiving portion 20a ofthe SOS sensor 20. The light receiving portion 20a is located at aposition equivalent to the image forming surface of the drum 1 in orderto detect the start position of image formation in the main scanningdirection. The incidental light to the light receiving portion 20a isadjusted by turning the inclination of the mirror 21 for the sensor inthe direction of the arrow E.

Meanwhile, in the optical apparatus such as disclosed in the presentinvention, the distance between the polygon mirror 12 and thephotoreceptor drum 1 becomes as long as 200 mm or more when the maximumimage width is set at the letter size or the legal size. In view of theforegoing, in the present embodiment, the image light path is madecompact and therefore the optical unit is made small by employing twoimage reflecting mirrors 14 and 15. As for the SOS light path, the lightpath is made compact and the optical unit is made small by reflectingthe beam twice by the first image reflecting mirror 14.

FIG. 4 shows a basic light path of the optical system with the mirrorsomitted therefrom. The beam deflected by the polygon mirror 12 by angleθ as maximum is turned by the fθ lens 13 in the direction θ'. Assumingthat the initial equivalent position of the photoreceptor is P, thescanning width of the photoreceptor surface 1a will be y1 for the angleof deflection θ. On this occasion, by moving the reflecting mirror 14 or15 in parallel to the light path, the distance between the deflector(polygon mirror) and the equivalent surface of the photoreceptor ischanged, whereby the scanning width (scanning line length) for thedeflection angle θ is changed.

The first mirror 14 is supported by a supporter 17 which is shiftablyprovided to the substrate 30. By the manipulation of the screw 18, thesupporter 17 is moved in parallel relative to the substrate 30 due to anelongated slit 19 and a screw 22.

The second mirror 15 has a similar construction to that of the firstmirror as described above.

Now, assuming that the equivalent position to the photoreceptor movesfrom P to Q by the distance Δx by the movement of the reflecting mirror14 or 15, the following equation is satisfied.

    y.sup.2 -y.sup.1 =2Δy=2Δx·tanθ' . . . (1)

As described above, the scanning line length can be adjusted by movingthe mirror 14 or 15, which accompanies the change of the position (anilluminating position) where the laser beam reaches the photoreceptordrum in a direction orthogonal to the drum axis. This change willappeared on the image as the change of the start position for imageformation.

Hereinafter explained is the changes of the length of the light path andthe illuminating position in the apparatus constructed such that a lightreflected by the second mirror does not intersect the light path fromthe polygon mirror 12 to the first mirror 14 by using two mirrors 14 and15.

Referring to FIG. 5, the laser beam from fθ lens 13 is reflected by thefirst mirror 14 which is inclined by an angle θ₁ in the clockwisedirection from a plane orthogonal to the beam scanning plane, and thebeam further is reflected by the second mirror 15 which inclines by anangle θ₂ in the counter-clockwise direction from the plane orthogonal tothe beam scanning plane (angle -θ₂ in the clockwise direction) tothereby reach the photoreceptor surface 1a. in this case, a reflectingangle of the first mirror 14 becomes 2θ₁, a reflecting angle of thesecond mirror 15 becomes

    2θ.sub.1 +θ.sub.2 +2θ.sub.1 +θ.sub.2 =4θ.sub.1 +2θ.sub.2,

and an incident angle to the photoreceptor surface 1a becomes 2θ₁ +2θ₂.

As shown in FIGS. 6(A) and 6(B), in the case where the first mirror 14is moved leftwardly from the position shown by a solid line to theposition shown by a chain line by the distance ΔX in FIG. 6(A), theamount of change of the light path Δ1, i.e., the amount of change of thescanning line length is represented as follows:

    Δ1=ΔX+AD-BC . . .                              (2)

Since the angles of inclination of the mirrors 14 and 15 to thesubscanning direction are defined as θ₁ and θ₂ respectively, thefollowing equations can be composed from the triangle ΔADB ##EQU1##

From the triangle ΔBCD, ##EQU2##

The amount of change of the scanning line length is obtained from theequations (2), (3) and (5): ##EQU3##

On the other hand, the amount of change α of the illuminating positionis represented as follows: ##EQU4##

Next, the amount of change Δ2 of the light path is represented asfollows in the case where the second mirror 15 is moved rightwardly fromthe position shown by a solid line to the position shown by a chain lineby the distance ΔX in FIGS. 7(A) and 7(B): ##EQU5##

On the other hand, the amount of change β of the illuminating positionis represented as follows: ##EQU6##

Each of the amounts of change mentioned above are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                               amount of change of                                           amount of change of                                                                           the illuminating                                              the light path  position                                               ______________________________________                                        movement of the mirror 14                                                               ##STR1##                                                                                        ##STR2##                                          movement of the mirror 15                                                               ##STR3##                                                                                        ##STR4##                                          ______________________________________                                    

The change rates of the illuminating position relative to the change ofthe light path are defined by the following equations wherein PM1represents the change rate with regard to the movement of the firstmirror 14 and PM2 represents the change rate with regard to the movementof the second mirror 15. ##EQU7##

If the ratio of these change rates is defined as γ, ##EQU8##

In the case where the ratio γ is more than "1", the movement of thefirst mirror 14 gives a greater amount of change of the illuminatingposition relative to the amount of change of the light path comparedwith the amount obtained by the movement of the second mirror 15.

The ratio γ of less than "1" brings the reverse result.

Next, there is explained the conditions required to the optIcalapparatus of the present invention where the first and second mirror 14and 15 are inclined by θ₁ and -θ₂ in the clockwise direction and a firstlight path between the polygon mirror 12 and the first mirror 14 doesnot intersected with a second light path between the second mirror 15and the photoreceptor 1. From FIG. 5,

    0°<2θ.sub.1 <90° , namely, 0°<θ.sub.1 <45°                                               (14)

    0°<4θ.sub.1 +2θ.sub.2 <180°, namely, 0°<2θ.sub.1 +θ.sub.2 <90°       (15)

    0°<2θ.sub.1 2θ.sub.2 <180°, namely, 0°<θ.sub.1 +θ.sub.2 <90°        (16)

A range fulfilling the conditions (14) to (16) is shown by a hatchportion in FIG. 8 of which abscissa and ordinate are θ₁ and θ₂respectively.

From the conditions (15) and (16),

    0°<θ.sub.1 +θ2<2θ.sub.1 +θ.sub.2 <90°

is introduced. This condition is transformed to the next condition (17)by subtracting θ₁ and 2θ₁ respectively, from this condition.

    -θ.sub.1 <θ.sub.2 <90°-2θ.sub.1 <90°. . . (17)

Consequently, the conditions required to the aforementioned opticalapparatus are summarized into the conditions (14) and (17).

With respect to the term (θ₁ +2θ₂), a following condition (18) isintroduced from the hatching range of FIG. 8.

    -45°<θ.sub.1 +2θ.sub.2 <180°     (18)

In the meantime, the ratio γ will be evaluated hereafter separately foran area (1) where 0°<θ₂ <90° and an area (2) where θ₂ <0°.

It is noted that the area (1) fulfills conditions 0°<θ₁ <45°, 0°<θ₂<90°, 2θ₁ +θ₂ 90°. To evaluate the ratio γ, the area (1) is furtherdivided into three areas (1a), (1b) and (1c) as shown more precisely inFIG. 9.

    0°θ.sub.1 +2θ.sub.2 ≦90°  area (1a)

    90°<θ.sub.1 +2θ.sub.2 ≦135°  area (1b)

    135°<θ.sub.1 +2θ.sub.2 ≦180°  area (1c)

Area (1a)

From the condition of the area (1a) and relations 0°<θ₁ <45° and 0°<θ₂<∵° which are known from FIG. 9, obtained is

sinθ₁ <sin(θ₁ +2θ₁).

it is determined for PM1 from the above condition and the equation (11)that:

    0<PM1<1.

On the other hand, it is determined for PM2 from the equation (12) andrelation

    sinθ.sub.2 <sin(2θ.sub.1 +θ.sub.2),

which is introduced the condition (15), that:

    PM2>1.

As a result, there is realized at the area (1a)

    0<γ<1.

Area (1b)

It is known from FIG. 9 that 0°<θ₁ <30°, 30°<θ₂ <67.5°. Further from thecondition of the area (1b),

    90°<θ.sub.1 +2θ.sub.2 ≦135°

    45°≦180°-(θ.sub.1 +2θ.sub.2)<90°

    0°<θ.sub.1 30°<45°≦180°-(θ.sub.1 +2θ.sub.2)<90°,

obtained is

    sinθ.sub.1 <sin{180°-(θ.sub.1 +2θ.sub.2)}=sin(θ.sub.1 +2θ.sub.2).

It is therefore determined for PM1 from the above condition and theequation (11) that

    0<PM1<1.

On the other hand, the amount of PM2 is more than 1 (PM2>1) as same asthe area (1a), so that there is realized at the area (1b)

    0<γ<1.

Area (1c)

It is known from FIG. 9 that 0°<θ₁ <15°, 60°<θ₂ <90°. The amount of PM2is more than 1 (PM2>1) as same as the areas (1a) and (1b).

The amount of PM1 can not be determined from the angular relations 0°<θ₁15°, 135°<θ₁ +2θ₁ ≦180°, so than another method is used where θ₁ iscompared with {180°-(θ₁ +2θ₂)}. From the condition of the area (1c),

    θ.sub.1 -{180°-(θ.sub.1 +2θ.sub.2)}=2θ.sub.1 +2θ.sub.2 -180°<0°.

Therefore,

    θ.sub.1 <180°-(θ.sub.1 +2θ.sub.2)

    sinθ.sub.1 <sin(θ.sub.1 +2θ.sub.2)

It is determined for PM1 that:

    0<PM1<1,

so that there is realized

    0<γ<1.

At the boundary between the areas (1) and (2) where θ₂ =0°, the amountof PM2 is infinity (PM2=∞). Accordingly, the light path can not bechanged by moving the second mirror 15.

Next, the ratio γ will be evaluated with regard to the area (2) where0°<θ₁ <45°, -45°<θ₂ <0°, θ₁ +θ₂ >0°, and 0°<2θ₁ +θ₂ <90°, -45°<θ₁ +2θ₂<45°. To evaluate the ratio γ, the area (2) is further divided intothree areas (2a), (2b) and (2c) as shown in FIG. 9 more precisely.

    -45°<θ.sub.1 +2θ.sub.2 <0 ° . . . area (2a)

    (-45°<θ.sub.2 <0°)

    θ.sub.1 +2θ.sub.2 =0°                   area (2b)

    0°<θ.sub.1 +2θ.sub.2 <45°        area (2c)

    (-22.5°<θ.sub.2 <0°)

Area (2b)

The area (2b) has an amount PM1=∞ and accordingly γ is infinity (γ=∞),that means there cannot be any change of the light path length by movingthe first mirror 14.

Areas (2a) and (2c)

From FIGS. 8 and 9, the conditions 2θ₁ +θ₂ >0°, θ₂ <0° are fulfilled atthe areas (2a) and (2c).

To evaluate the ratio γ, a first method is used where (2θ₁ +θ₂) iscompared with (-θ₂).

    2θ.sub.1 +θ.sub.2 -(-θ.sub.2)=2θ.sub.1 +2θ.sub.2 >0°

    2θ.sub.1 +θ.sub.2 >-θ.sub.2 >0°

    sin(2θ.sub.1 +θ.sub.2)>sin(-θ.sub.2)>0°

    {sin(2θ.sub.1 +θ.sub.2)/sin(-θ.sub.2)}>1

    PM2 >1

It is therefore determined for the areas (2a) and (2c) that:

    PM2 <-1 . . .                                              (21)

For PM1 in the area (2a), (θ₁) is compared with {-(θ₁ +2θ₂)}.

    θ.sub.1 -{-(θ.sub.1 +2θ.sub.2)}=2θ.sub.1 +2θ.sub.2 ·>0°

Therefore,

    PM1< -1 . . .                                              (22)

For PM1in the area (2c), (θ₁) is compared with (θ₁ +2θ₂).

    (θ.sub.1 -(θ.sub.1 +2θ.sub.2)+-2θ.sub.2 >0°

    {sinθ.sub.1 /sin(θ.sub.1 +2θ.sub.2)}>1

Therefore,

    PM1>1 . . .                                                (23)

As apparent from the conditions (21), (22), (23), it is impossible todetermine whether or not the ratio |γ| is larger than 1.

Next, a second method is used where combinations of terms included inthe numerator and denominator of γ are estimated. The term combinationsother than the term combinations of PM1and PM2 are as follows:

    sinθ.sub.1 /sin(2θ.sub.1 +θ.sub.2) . . . (24)

    sinθ.sub.2 /sin(θ.sub.1 +2θ.sub.2) . . . (25)

From the relation θ₁ >0°, 2θ₁ +θ₂ >0°,

    (2θ.sub.1 +θ.sub.2)-(θ.sub.1)=θ.sub.1 +θ.sub.2 >0 °

    0°<θ.sub.1 <2θ.sub.1 +θ.sub.2 <90°

    0°<sinθ.sub.1 <sin(2θ.sub.1 +θ.sub.2)

The expression (24) is therefore less than 1, namely,

    sinθ.sub.1 /sin(2.sub.1 +θ.sub.2)<1 . . .      (24')

For the expression (25), (-θ₂) is compared with {(θ₁ +2θ₂)} with respectto the area (2a). ##EQU9## As apparent from the expressions (24') (25'),it is impossible to determine the value of the ratio γ at the area (2a)by use of the second method.

With respect to the area (2c), (-θ₂) is compared with (θ₁ +2θ₂).

    (-θ.sub.2)-(θ.sub.1 +2θ.sub.2)=-(θ.sub.1 +3θ.sub.2)

If (θ₁ +3θ₂) is equal to or more than 0°,

    0°<-θ.sub.2 ≦θ.sub.1 2θ.sub.2 <45°,

so that the expression (24) becomes equal to or less than 1, namely,

    |sin(-θ.sub.2)/sin (θ.sub.1 +2θ.sub.2)|≦1.

Therefore, the absolute value of the ratio γ becomes less than 1,namely,

    |γ|<1,

when θ₁ +3θ₂ ≧0 which is shown in FIG. 10 by the hatch portion.

Or the contrary, if (θ₁ +3θ₂) is negative, it is impossible to determinethe value of the ratio γ.

Third method is attempted to evaluate the value of the ratio γ. Thethird method uses the following equation which obtained from theequations (11) and (12): ##EQU10## The numerator of the equation (26) istransformed as follows: ##EQU11##

The area (2a) has a condition θ₁ +2θ₂ <0°, θ₁ >0° and therefore PM1<-1and PM2 <-1. Further, the area (2a) has a condition,

    0°<θ.sub.1 +θ`<22.5°,

which is known from FIG. 11.

    0 <θ.sub.1 +θ.sub.2 <3(θ.sub.1 +θ.sub.2)<67.5°

    cos(θ.sub.1 +θ.sub.2)>cos3(θ.sub.1 +θ.sub.2)

Accordingly, the numerator of [PM1-PM2]<0.

On the other hand, the denominator of (PM1-PM2) is positive since (θ₁2θ₂) and (θ₂) are both negative. Therefore, the following relation isintroduced:

    PM1-PM2<0

    PM1<PM2<0

    γ=(PM1/PM2)>1 . . .                                  (28)

On the contrary, it is imPossible for the area (2c) to determine thevalue of the ratio γ by using the third method, because of theimpossible of the transformation of an expression (PM1+PM2) which isrequired to evaluate the ratio γ.

The evaluation result up to the present is made up into the followingtable 2 for the optical apparatus fulfilling the conditions (14) and(17).

                  TABLE 2                                                         ______________________________________                                                   PM1         PM2        γ                                     ______________________________________                                        θ.sub.2 > 0°                                                                0 < PM1 < 1 PM2 > 1    0 < γ < 1                             θ.sub.2 = 0°                                                                1           PM2 = ∞                                                                            γ = 0                                 θ.sub.2 < 0°                                                     θ.sub.1 + 3θ.sub.2 ≧ 0°                                        PM1 > 1     PM2 < -1   -1 < γ < 0                            θ.sub.1 + 3θ.sub.2 < 0°                                               PM1 > 1     PM2 < -1   indeter-                                    θ.sub.1 + 2θ.sub.2 > 0°                                                                      minable                                     θ.sub.1 + 2θ.sub.2 = 0°                                               PM1 = ∞                                                                               --       γ = ∞                           θ.sub.1 + 2θ.sub.2 < 0°                                               PM1 < -1    PM2 < -1   γ  > 1                                ______________________________________                                    

As apparent from the table 2, the ratio is classified into three regionsA, B and C as shown in FIG. 12.

The region A is denoted by the right-inclined hatching and fulfills thecondition θ₁ +3θ₂ ≧0° in addition to the conditions (14) and (17). Inthe region A, the first mirror 14 is moved for adjusting the scanningline length to thereby obtain smaller aberration of the illuminatingposition than a case where the second mirror 15 is moved.

The region C is denoted by cross-hatching and fulfills the condition θ₁+2θ₂ ≦0° in addition to the conditions (14) and (17). In the region Cthe second mirror 15 is moved for adjusting the scanning line length tothereby obtain smaller aberration of the illuminating position thanmoving the first mirror 14.

In the region B denoted by left-inclined hatching and having theconditions θ₁ +2θ₂ >0° and θ₁ +3θ₂ <0°, it is indeterminable that eitherthe first or second mirror is to be moved for adjustment, so that thescanning line length is adjusted by a manner of trial and error.

As apparent from the above, the apparatus according to the presentinvention affords a lesser shift amount of the illuminating position onthe recording medium when the scanning line length is adjusted by movingthe first or second mirror.

Although the present invention has been fully described by wa ofexamples with reference to the accompanying drawings, it is to be notedthat various changes and modifications will be apparent to those skilledin the art. Therefore, unless otherwise such changes and modificationsdepart from the scope of the present invention, they should be construedas being included therein.

What is claimed is:
 1. A laser beam scanning apparatus for beam scanninga medium with a laser beam being deflected by a deflector, comprising:afirst mirror provided with a surface for reflecting the laser beam,after being deflected by the deflector along a first light path; asecond mirror with a surface provided for reflecting the laser beam fromthe first mirror for folding a light path between the first mirror andthe medium in such a relationship that the first light path between thedeflector and the first mirror is not intersected with a second lightpath between the second mirror and the medium, and the first and secondmirrors being positioned so as to fulfill the following conditions:

    0<θ.sub.1 <45°

    -θ.sub.1 <θ.sub.2 <90°-2θ.sub.1 <90°

where θ₁ and θ₂ represent angles between respectively the first andsecond mirror surfaces thereof and a plane perpendicular to a beamscanning plane from the deflector to the first mirror, the first andsecond mirrors are movable, respectively, for adjusting a length of ascanning line on the medium, wherein when one of the first and secondmirrors is moved during adjusting, said moved mirror has a lesser shiftamount of the scanning line on the medium in a direction perpendicularto the scanning line as compared with a movement of the other mirror. 2.A laser beam scanning apparatus as claimed in claim 1, wherein the firstmirror is moved for adjusting the length of scanning line when(PM1/PM2)<1 and the second mirror is moved for adjusting the length ofscanning line when (PM1/PM2)<1, where PM1 represents the change rate ofthe shift amount of the scanning line with regard to the movement of thefirst mirror and PM2 represents the change rate of the shift amount ofthe scanning line with regard to the movement of the second mirror. 3.In a laser beam scanning apparatus for beam scanning a medium with alaser beam being deflected by a deflector through mirrors, a method forproviding specific positional relationship to the mirrors for adjustingas scanning line comprising in steps for;providing a first mirror fordeflecting the laser beam after deflection by the deflector and thesecond mirror for reflecting the laser beam from the first mirror forfolding a light path between the first mirror and the medium; arrangingthe first and second mirrors in such a relationship that a first lightpath between the deflector and the first mirror is not intersected witha second light path between the second mirror and the medium and thatthe following conditions are fulfilled:

    0<θ.sub.1 <45°

    -θ.sub.1 <θ.sub.2 <90°-θ.sub.1 <90°

where θ₁ and θ₂ represent angles between the mirror surfaces thereof anda plane perpendicular to a beam scanning plane from the deflector to thefirst mirror; and moving either one of the first and second mirrors inorder to adjust a length of scanning line on the medium in such a mannerthat said moved mirror has a lesser shift amount of the scanning line onthe medium in a direction perpendicular to the scanning line as comparedwith the other mirror.
 4. A method for providing positional relationshipto the mirrors as claimed in claim 3, wherein the first mirror is movedfor adjusting the length of scanning line when (PM1/PM2)<1 and thesecond mirror is moved for adjusting the length of scanning line when(PM1/PM2)>1, where PM1 represents the change rate of the shift amount ofthe scanning line with regard to the movement of the first mirror andPM2 represents the change rate of the shift amount of the scanning linewith regard to the movement of the second mirror.